The present invention relates to slip assemblies used in the oil and gas drilling industry. In particular, the present invention relates to an improved slip surface which allows the die carriers of the slip assembly to apply greater radial force to the tubular member being gripped.
Various types of slip assemblies are known in the art. U.S. Pat. No. 4,681,193 to Crowe discloses a typical slip assembly which is operated with hydraulic cylinders. The Crowe slip assembly has a slip bowl with an open top and bottom and which has an inwardly sloping slip surface of a continuous curvature around the inside parameter of the bowl. In essence, the slip bowl forms a funnel shaped slip surface. A plurality of slip die carriers (e.g. three) are designed to fit within the slip bowl. Each of the die carriers will include a sloping arcuate surface which has a curvature corresponding to the curvature of the bowl""s slip surface. However, it will be understood that this correspondence between the slip bowl""s surface and the die carrier""s slip surface occurs only at a single location on the slip bowl. As is well known in the art, as the die carriers ride down the bowl""s sloping slip surface, the die carriers are moved radially inward in order to engage a tubular member projecting through the center of the bowl. Likewise, raising the die carriers in the bowl allows the die carriers to move away from the tubular, thereby releasing the tubular. Typically, slip assemblies are employed in conjunction with a secondary type of tubular gripping and lifting device. The lifting device will grip and lift the tubular member. The slip assembly with then engage the tubular member so that the lifting device may release the tubular member and grip the tubular member in a lower position in preparation for another lift.
It is common in the drilling industry to handle tubulars having slight variations in diameter do to machining tolerances, scarring on the tubular""s outer skin, or other wearing of the tubular surface. While these variations are not great in magnitude, they do often create a problem in relation to the prior art slip assembly. The prior art does allow for the use of different die carriers for different standard tubular diameters. However, because the slip surface of the prior art bowl is in essence funnel shaped, the tubular must be virtually the exact standard diameter in order to allow the die carrier""s rear surface to perfectly match the bowl surface along the entire slope of the slip surface. Nevertheless, there is almost always some variations in diameter from tubular to tubular. This results in the die carriers not uniformly contacting the slip bowl, thus resulting in die carriers not applying uniform force to the tubulars or the die carriers having a tendency to xe2x80x9crockxe2x80x9d in the slip bowl. Both of these problems are detrimental to the effective and non-damaging gripping of tubulars.
Another disadvantage of prior art slip bowls is the comparatively high coefficient of friction (COF) between the die carrier""s and the bowl""s slip surfaces. Viewing FIG. 1A, slips may be conceptualized as two inclined planes sliding against one another. Block 4A would represent the slip bowl surface and block 4B would represent the inclined surface on the die carrier. The angle alpha (xcex1) of the slip surface seen in FIG. 1A will typically be approximately 80 degrees. It will be understood that the force generated by the COF (Ff in FIG. 1A) has a component (Fx) which acts in the opposite direction of the radial force (FA) used to grip the tubular. Therefore, the higher the COF on the slip surface, the lower the amount of radial force available for the die carrier to utilize in gripping the tubular. Normally, the COF of this steel on steel contact is approximately 0.08. It would be a significant advance in the art to provide a slip assembly which substantially reduced the COF on the slip surfaces and applied more gripping force to the tubular member.
It would also be advantageous to supply an improved slip assembly which would allow the slip assembly to be mounted on a rotary table or the like and to provide rotational force or torque to the tubular member by way of the slip assembly. This is not easily carried out with the prior art slip assemblies such as seen in the Crowe reference because the die carriers are not firmly fixed in the slip bowl against lateral movement as torque is applied.
The present invention comprises an improved slip assembly. The slip assembly has a base and at least two opposing slip frames positioned on the base with each of the slip frames including a planar slip surface. Additionally, a die is carrier positioned within each of the slip frames and each of the die carriers also includes a planar surface which engages the slip surfaces of the slip frames.
The present invention also includes a low friction slip assembly having a base and at least two opposing slip frames positioned on the base. Each of the slip frames will include a slip surface having an effective coefficient of friction less than about 0.07 and die carriers will be position within each of the slip frames.
The present invention further includes an improved slip assembly which has a base plate with a center aperture formed therein. There will be at least two separate slip frames positioned around the center aperture and each of the slip frames will include a slip surface. A die carrier will be positioned within each of the slip frames and each of the die carriers will include a surface for engagement with the slip surfaces of the slip frames.